We follow the notation and conventions adopted in
[1]}. In particular, the Minkowski metric is
\(\eta _{ab}=\mbox{diag}(-1,1,1)\) .
The spinor indices are raised and lowered using
the \(\rm SL(2,{\mathbb {R}})\)  invariant tensors
\(\varepsilon _{\alpha \beta }=\left(\begin{array}{cc}0~&-1\\1~&0\end{array}\right)~,\qquad \varepsilon ^{\alpha \beta }=\left(\begin{array}{cc}0~&1\\-1~&0\end{array}\right)~,\qquad \varepsilon ^{\alpha \gamma }\varepsilon _{\gamma \beta }=\delta ^\alpha _\beta \) 
